drm: rcar-du: Output the DISP signal on the ODDF pin

The ODDF signal, output by default on the ODDF pin, isn't used on any
board supported in the kernel. As the Gen3 Salvator-X board uses the
ODDF pin as a DISP signal, hardcode that configuration in the driver.

Use of the ODDF signal will be implemented later through proper DT-based
configuration of the DU pins.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>